1. Field of the Invention
The present invention relates to a stirring device for a test of a specimen, which mixes and stirs reaction solutions including reagents and specimens such as serums.
2. Description of the Related Art
In a test of a specimen, when the specimen is reacted with a reagent, it is an indispensable technique in obtaining reaction reproducibility, to stir and homogenize a reaction solution consisting of the specimen and the reagent. High-speed operation of an automatic apparatus (biochemical analyzer) for a test of a specimen has recently been developed, but there is a problem. That is, a predetermined length of time is required for stirring the reaction solution.
An example of a conventional stirring method is a method of stirring a reaction solution by using a motor. FIG. 1 shows an example of this method. A solution in a vessel 10 is stirred by an impeller 12 attached to an end portion of a rotating shaft of a small-sized motor 9.
FIG. 2 shows another example. A freely movable magnet 13 is placed within the vessel 10. A piece of a magnetic metal 9a below the vessel 10 is rotated by the motor 9. By utilizing an attraction force of the magnet 13, the magnet 13 within the vessel 10 is rotated in accordance with the rotation of the piece of a magnetic metal 9a, thereby stirring the solution.
In these methods, stirring is effected in a two-dimensional manner at the bottom part of the vessel 10, and the stirring effect does not act at the upper part of the vessel. Thus, an efficiency of the stirring is low and a long time is required for stirring the entire solution. For example, four seconds or more were required for homogenizing a reaction solution of 400 to 600 .mu.l. A stirring performance CV (coefficient of variance=(standard deviation/average value).times.100%) of the stirring device of rotating type as shown in FIGS. 1 and 2 were tested by using a biochemical analyzer for measuring the amount of protein in a serum of 8 .mu.l. The contents of the reaction solution were 308 .mu.l and the number of measurements was 30. The CV was 0.68% for the stirring time of 2 seconds and 0.23% for the stirring time of 4 seconds.
Since the stirring incurs rotation of solution, the solution within the vessel 10 is rotated in a laminar flow, as shown by a broken line in FIGS. 1 and 2, resulting in a longer time for stirring. If the rotation speed of the motor 9 is increased to shorten the stirring time, bubbling occurs, or air (air bubbles) is taken in. Depending on the shape or size of the vessel, solution may spill out of the vessel 10 or be splashed.
An example of a conventional stirring method in which no motor is used is disclosed in U.S. Pat. No. 4,612,291. According to this method, as shown in FIG. 3 (front view) and FIG. 4 (side view), piezoelectric elements 18a and 18b are attached to both surfaces of a metal thin plate 15, thus constituting a bimorph-type piezoelectric vibrator. A distal end portion of the metal thin plate 15 of the piezoelectric vibrator is connected to a stirring rod 14 of a solid structure made of, e.g., stainless steel. A proximal end portion 17 of the metal plate 15 is fixed to a support (not shown). As is shown in FIG. 4, an AC voltage 16 is applied to the piezoelectric elements 18a and 18b, and the piezoelectric elements 18a and 18b are alternately vibrated and the stirring rod 14 is vibrated as shown by broken lines in FIG. 4. By vibration of the stirring rod 14, the solution within the vessel 10 can be stirred. Since the stirring rod 14 is a solid body, the rod 14 as a whole vibrates in a primary mode.
According to this method, since the amplitude of vibration of the stirring member 14 is proportional to the applied voltage, the stirring time can be decreased by increasing the voltage. However, if the amplitude is greatly increased, the stress of the proximal end portion 17 of the piezoelectric vibrator increases, with the result that the piezoelectric vibrator may be damaged or the connecting portion between the metal plate 15 and the stirring rod 14 may be mechanically broken. Since the stirring rod 14 is placed in the vessel with a fixed width, the vessel may be broken by the vibration of the stirring rod if the amplitude of the vibration greatly increases. Thus, the effect of the stirring is limited. In addition, since the vibration mode is the primary mode, the entire reaction solution is not stirred. Thus, the stirring efficiency is low in this method, as with the case of using the motor.